US6134039A - Wavelength dependent thermally compensated optical system - Google Patents

Wavelength dependent thermally compensated optical system Download PDF

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Publication number
US6134039A
US6134039A US09/222,146 US22214698A US6134039A US 6134039 A US6134039 A US 6134039A US 22214698 A US22214698 A US 22214698A US 6134039 A US6134039 A US 6134039A
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thermal
lens
focusing
focusing lens
diffractive
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Expired - Lifetime
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US09/222,146
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Robert W. Rudeen
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Datalogic Scanning Inc
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PSC Scanning Inc
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Priority to US09/222,146 priority Critical patent/US6134039A/en
Priority to EP99100879A priority patent/EP0932113A3/de
Priority to JP11018410A priority patent/JPH11271659A/ja
Assigned to PSC SCANNING, INC. reassignment PSC SCANNING, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: RUDEEN, ROBERT W.
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Publication of US6134039A publication Critical patent/US6134039A/en
Assigned to FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT reassignment FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT SECURITY INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: OPTEL SYSTEMS LIMITED, OPTICAL STORAGE INTERNATIONAL, PSC AUTOMATION ( FORMERLY LAZERDATA CORPORATION), PSC SCANNING, INC. (FORMERLY SPECTRASACN, INC., SPECTRA-PHYSICS SCANNING SYSTEMS, INC. AND PSC ACQUISITION, INC., PSC, INC.
Assigned to PSC INC. reassignment PSC INC. RELEASE OF SECURITY INTEREST Assignors: LJ SCANNER HOLDINGS, INC., AS SUCCESSOR IN INTEREST TO FLEET NATIONAL BANK (A/K/A FLEET BANK), AS ADMINISTRATIVE AGENT
Assigned to WELLS FARGO FOOTHILL, INC. reassignment WELLS FARGO FOOTHILL, INC. SECURITY INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: PSC SCANNING, INC.
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    • G—PHYSICS
    • G06—COMPUTING OR CALCULATING; COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
    • G06K7/10—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation
    • G06K7/10544—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation by scanning of the records by radiation in the optical part of the electromagnetic spectrum
    • G06K7/10792—Special measures in relation to the object to be scanned
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S359/00—Optical: systems and elements
    • Y10S359/90—Methods

Definitions

  • the field of the present invention relates to data reading devices such as for example bar code scanning systems.
  • the invention is applicable to both stationary or handheld scanners.
  • a light source such as a laser, laser diode, or non-coherent light source (e.g. light emitting diode) emits light which passes through and is focused by the focusing lens system.
  • the object to be scanned is passed through the focused beam and if the bar code is sufficiently close to the beam focal point, reflected light from the bar code may be detected resulting in a successful scan.
  • the scanning device may comprise moving spot laser beams from a laser or laser diode with the beam(s) scanned in one or more various directions out through the scanner window, CCD's, other imaging devices or a non-moving beam, such as a wand, in which the wand is moved across the bar code symbol or the object itself is moved past the beam in order to sweep the code symbol.
  • a non-moving beam such as a wand
  • data reading devices such as bar code scanners depend upon focused optics for effective and accurate performance.
  • a focal point is typically not a discrete point but may be referred to as a "waist.”
  • the waist is the position along the beam axis where the "cone" of light from the light source reaches a minimum spot size, usually as measured in a direction parallel to the direction of spot motion.
  • the checkout scanner is designed with a scanning beam with a waist of a given diameter positioned at a certain distance from the window where the bar code is expected to pass.
  • the checkout clerk generally becomes familiar with the proper distance to pass the object in front of the window, that is, the bar code must pass sufficiently close to the scanner focal point or waist (i.e. within its depth of field) in order to achieve a successful scan.
  • the diode produces a laser beam which is focused by a glass lens.
  • the lens barrel of the laser diode in the PSC Inc. model VS-1200TM bar code scanner is designed/optimized to offset the focal shift due to the thermal expansion of the glass lens.
  • the present inventor has determined that temperature may have a significant impact on the focusing properties of the reader and thus the focal distance when a plastic lens is used. Depending on the application, a scanner may experience significant temperature variation and thus produce a significant impact on focusing.
  • the present inventor has recognized that several scanner components are affected by temperature including:
  • a scanner includes a plastic focusing lens and thermal compensation element, integrally formed with the asphere or comprising a separate element, to replace the existing glass laser diode focusing lens.
  • the combined optical system is thereby configured to provide overall temperature compensation for the system with reduced cost and improved performance.
  • FIG. 1 is a schematic of a data reader system according to a preferred embodiment
  • FIG. 2 is a graph of beam propagation versus temperature for a plastic asphere
  • FIG. 3 is a graph of beam propagation versus temperature for a plastic asphere and temperature compensation element
  • FIG. 4 is front elevation view of an integrated focusing lens and temperature compensation optical element
  • FIG. 5 is a cross sectional view of the optical element of FIG. 4.
  • Diffractive optics are different from conventional (refractive) optics in that they do not rely on surface curvature to change the phase of the wavefront.
  • Geometric raytracing as defined by Snell's Law, describes how light entering or exiting a curved surface will change direction at the surface.
  • a diffractive surface has small regions or steps (flat or curved) which look like a microscopic fresnel lens. However, the period of these steps are on the order of the wavelength of the light so they do not obey Snell's Law (periods on fresnel lenses are hundreds of wavelengths).
  • the steps can be designed to produce the same wavefront as the refractive surface, however the maximum step height changes the phase by only 2 ⁇ radians or ⁇ /(n-1) in units of length.
  • the element will redirect the light in the same way as the refractive-equivalent, but it is caused by diffraction of light exiting the steps rather than refraction. This difference means that the wavefront produced by a diffractive surface is more sensitive to wavelength than a refractive element and less sensitive to temperature.
  • the focal distance of lenses themselves may vary with temperature with plastic (molded) lenses generally varying to a somewhat greater degree than glass lenses. It has also been determined that laser diode beam frequency (wavelength) varies with temperature, and since the focal distance of many diffractive or refractive lenses vary with the frequency of the beam, temperature also impacts focal distance due to the wavelength shift. Focus variations due to thermal effect on plastic (i.e. molded) lenses are a problem for electro-optical systems such as bar code scanners.
  • a conventional refractive lens is affected by temperature changes in two ways.
  • the physical dimensions (radius of curvature, thickness, diameter) change according to the coefficient of thermal expansion.
  • This coefficient ( ⁇ ) is about 7 to 10 ⁇ 10 -6 /° C. for glass (e.g. BK-7) and 60 to 90 ⁇ 10 -6 /° C. for plastic (e.g. polystyrene).
  • the focal length is proportional to the radius of curvature so it increases linearly with temperature.
  • a plastic lens will have a focal length shift of roughly 10 ⁇ that of a glass lens due to this phenomenon.
  • the other affect of temperature is to change the index of refraction of the optical element (e.g. lens) material.
  • the coefficients that relate to index change are much greater for plastic than for glass. Since the deviation of the ray angle is related to the index by Snell's Law, this temperature factor also effects the lens focal length.
  • n Index of refraction of the media
  • n 0 Index of refraction of air.
  • a diffractive surface is defined with a polynomial which yields the phase of the wavefront as a function of location on the lens. The overall diameter will increase and decrease with temperature which will scale this polynomial.
  • the focal length of a diffractive element is not dependent on the index of refraction of the media. Only the index of the media where the waist occurs is relevant (i.e. air in most cases) because a diffractive element is essentially a zone plate. Since the index of air is close to 1 and changes very little over temperature, temperature effect on a diffractive element has a relatively small contribution toward focal length shift.
  • the diffractive element is twice as sensitive to thermal expansion as the refractive element. However, since there is no dn/dt dependence, and dn/dt is the major contributor to focal shift for plastic optics, the diffractive element has a lower shift overall. Table 1 below shows some results based on the Equations 1 and 2 above. These focal length shifts may not seem very large until it is noted that a typical scanner optical system has a longitudinal magnification factor of ⁇ 20. This factor yields a waist movement of ⁇ 46% of the image distance for a polystyrene refractive lens.
  • the final contributor to the shift in the waist location is the thermal expansion of the optics assembly mount 25.
  • the effect of this contributor is to change the working distance of the lens.
  • the actual shift is dependent on the materials used and the attachment locations.
  • Refractive optical elements are effected by the wavelength shift through a change in the index of refraction.
  • the focal shift due to wavelength shift is much smaller than the thermal effects (0.10% vs 2.3% over the wavelength shift that occurs between 0° C. and 50° C.).
  • DOE diffractive optical element
  • n dielectric refractive index
  • a lens may be designed according to a preferred embodiment having a refractive optical surface and a diffractive optical surface.
  • FIG. 1 illustrates an optical system 10 comprising a laser diode 12 and a focusing lens 16.
  • the focusing lens 16 and the laser diode 12 are assembled in an optics assembly mount or barrel 25 to form a visible laser diode module (VLDM).
  • VLDM visible laser diode module
  • FIG. 1 illustrates that the focal point of the system 10 may vary due to temperature, with focal distances illustrated at three different temperatures (0° C., 20° C. and 40° C.).
  • FIG. 1 illustrates a preferred embodiment by the addition of a thermal compensation optical (TCO) comprising a thermal correction (or compensation) plate 20 (TCP) positioned in front of the focusing lens 16.
  • TCO thermal compensation optical
  • TCP thermal correction (or compensation) plate 20
  • the TCP 20 is a separate optical element positioned downstream of the focusing lens 16.
  • the TCP 20 may be incorporated into the VLDM and mounted within the barrel 25, or may be mounted outside the barrel at another location within the scanner.
  • the TCP 20 will be located near the VLDM to minimize its size and therefore its cost, but conceivably the TCP could be incorporated into other elements such as for example the scanning mechanism (e.g., the scanning mirror 30) or the scanner window.
  • the thermal compensation optics may comprise a single or multi-component TCP but preferably a single component such as a diffractive optic element (DOE) plate as described below.
  • DOE diffractive optic element
  • a DOE plate was designed which compensated for the thermal shift of an off-the-shelf acrylic asphere (9 mm EFL).
  • a lens manufacturer in the first instance Digital Optics Corporation, Charlotte, N.C. USA was provided with specifications to construct a test thermal compensation plate which would have defined optical characteristics shifting in focal length due to the change in laser diode wavelength.
  • the manufacturer designed the TCP optics model using Code VTM, an optical design software package commercially available from Optical research Associates, Pasadena, Calif. USA. This design was then verified on ZemaxTM also an optical design software package commercially available from Focus Software Inc., Arlington, Ariz. USA.
  • the DOE had some optical power which made the EFL of the entire system about 7.2 mm.
  • the optics assembly mount was included in the model. This model predicted thermal shift of the waist location of 0.4% from 0° C. to 50° C. An equivalent acrylic lens would yield a shift of 27%. An equivalent BK-7 lens would have a shift of 1.3%.
  • This thermal compensation plate 20 (TCP) was then fabricated on a silica substrate and tested in the lab with the acrylic asphere 16. Both optical elements were mounted into one lens barrel 25 which was mounted into the laser diode housing and focused. The entire optics module 10 was then placed inside a small thermal chamber. The beam diameter as a function of distance was measured at room temperature, 50° C. and 60° C. All temperature measurements were taken on or very near ( ⁇ 5 mm) the lens barrel 25.
  • FIG. 2 illustrates graphs of Beam Diameter versus Distance from the target.
  • FIG. 2 shows the beam propagation for the perpendicular and parallel axes at 28.2° C., 50.3° C. and 64.5° C. for the case of the acrylic asphere without the TCP.
  • the parallel waist shifts by 23 mm which correlates with the ZemaxTM model.
  • the perpendicular axis is truncated which reduces the thermal shift.
  • ZemaxTM does not model the effects of beam truncation.
  • FIG. 3 shows the beam propagation with the TCP in place.
  • the shift in waist location between 26.3° C. and 61.1° C. is only 2 mm.
  • the ZemaxTM model predicted 3 mm. It is believed that the reason the compensation was not perfect was two-fold.
  • the TCP was etched into a substrate with a different thickness than was used in the design.
  • the separation between the asphere and the TCP was different in the as-built case from the design. When these discrepancies were included in the ZemaxTM model, the resulting 3 mm shift was predicted thus providing good correlation with the experimental results.
  • FIGS. 4-5 illustrate a combination lens 50 including a thermal compensation element 55 incorporated into a plastic focusing lens element 60. All dimensional units in FIGS. 4-5 are in mm.
  • the focusing lens element 60 includes an asphericle optical surface 62 designed to focus the laser beam at a given distance.
  • the optical surface 62 is defined by:
  • the diffractive optical surface 57 is constructed with a phase wavefront defined by:
  • the effective focal length is 6 mm at a nominal wavelength of 679 nm.
  • the lens material is Dow 685D polystyrene.
  • Other plastic materials may be employed such as acrylic, but would require some lens redesign to accommodate the change in material.
  • the diffractive and refractive surfaces may be combined one optical surface.
  • the diffractive "ridges" are superimposed on the curvature of the diffractive surface.
  • This structure may have the advantage of simplifying fabrication since only one surface must be diamond turned. For lenses of different focal length, only one surface need be modified.
  • Additional savings may potentially be realized by incorporating an aperture into the lens and/or implementing a press-fit flange on the lens. Further combinations of optical elements (integral and/or separate) may be implemented such as those described in U.S. Pat. No. 5,565,668 herein incorporated by reference.
  • a computer model is only as good as the input data.
  • the above analysis has used data on the plastic materials from the manufacturer and have called out specific materials on the drawing. However, we have not verified the thermal coefficients for dn/dt experimentally. Because of the hydroscopic properties of acrylic, plastic lens made from polystyrene may be used which has a much lower focal length shift as humidity changes. The experiment described above was for an acrylic lens.
  • the information on the wavelength vs. temperature characteristics of the 1131VS laser diode was obtained from Sony Corporation which indicated that the slope can shift between 0.10 nm/° C. and 0.18 nm/° C. Putting these values into the ZemaxTM model yields a worse case shift in waist position of 8% between 0° C. and 50° C. This shift corresponds to 10 mm for a scanner such as the PSC Inc. model QS6000 bar code scanner. The value of this slope may be measured with the ILX Laser Diode Measurement System available from ILX Lightwave, Bozeman, Mont., USA.
  • Injection molding of DOE's is still a new technology.
  • First article testing can address the fabrication and replication issues, but tool wear should be addressed with a more aggressive maintenance procedure and fabrication of back-up DOE pins.
  • the fabrication process for the pins may allow extras to be made at the same time so back-ups can be available.
  • Diffraction efficiency testing can be done on a lot to lot basis to quantify tool wear.

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  • Electromagnetism (AREA)
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US09/222,146 1998-01-27 1998-12-29 Wavelength dependent thermally compensated optical system Expired - Lifetime US6134039A (en)

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US09/222,146 US6134039A (en) 1998-01-27 1998-12-29 Wavelength dependent thermally compensated optical system
EP99100879A EP0932113A3 (de) 1998-01-27 1999-01-19 Wellenlängenabhängiges thermisch kompensiertes optisches System
JP11018410A JPH11271659A (ja) 1998-01-27 1999-01-27 波長により温度補正する光学システム

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US20030201319A1 (en) * 1998-10-19 2003-10-30 Mehul Patel Optical code reader for measuring physical parameters of objects
US6678044B2 (en) * 2000-07-17 2004-01-13 Ando Electric Co., Ltd. Monochromator and optical spectrum analyzer equipped with the same
US6693745B1 (en) * 1999-09-14 2004-02-17 Corning Incorporated Athermal and high throughput gratings
US20040070769A1 (en) * 2002-07-05 2004-04-15 Carsten Dam-Hansen Temperature compensated optical system based on a diffractive optical element
US6775065B2 (en) * 2000-10-06 2004-08-10 Kabushiki Kaisha Sankyo Seiki Seisakusho Diffraction element and optical pickup device
US20050078775A1 (en) * 2002-09-23 2005-04-14 Martin Hellmark Mitigating the impact of phase steps
CN103852835A (zh) * 2012-12-06 2014-06-11 三菱电机株式会社 光模块及光传送方法
DE102014114253A1 (de) 2013-09-30 2015-04-02 Jenoptik Optical Systems Gmbh Verfahren zur Kompensation von Abbildungsfehlern durch Effekte thermischer Linsen in einer Optik
US9187360B2 (en) 2012-04-20 2015-11-17 Schott Corporation Glasses for the correction of chromatic and thermal optical aberations for lenses transmitting in the near, mid, and far-infrared sprectrums

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JP5186091B2 (ja) * 2006-06-23 2013-04-17 株式会社ミツトヨ 光電式エンコーダ
JP5024928B2 (ja) * 2006-09-04 2012-09-12 株式会社リコー 光走査装置及び画像形成装置
US8840323B2 (en) 2012-08-27 2014-09-23 Avago Technologies General Ip (Singapore) Pte. Ltd. Thermally compensated parallel optical communication module
DE102019107146B4 (de) 2019-01-30 2021-02-25 Jenoptik Optical Systems Gmbh Athermale Laseroptik aus Kunststoff
CN114136463B (zh) * 2021-11-30 2023-07-25 湖北久之洋信息科技有限公司 一种基于红外热像仪的自动对焦标定方法、装置及系统

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EP0932113A2 (de) 1999-07-28
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